| Carbon nanotubes(CNTs)exhibit outstanding mechanical,electrical,and thermal properties.Considerable efforts have been devoted to assembling nanoscopic CNTs into macroscopic materials that are suitable for numerous advanced applications,such as CNT fibers.However,CNTs cannot be dissolved and melted.Moreover,due to very short length(<50 μm),most commercially available CNT raw materials exist in the formation of powder,and tend to aggregate easily during the dispersion process.Assembling CNTs into one-dimensional macroscopic continuous materials with wellmaintained properties of single CNT at the nanoscale has long been a quite challenging issue.A simple and direct fiber processing approach is to dope CNTs into polymers by either solution or melt spinning.Due to the limited content of introduced CNTs,however,the electrical/thermal properties of finally formed CNTs doped polymer fibers are usually poor.An effective strategy for CNT fibers with good electrical / thermal conductivity is to increase the content of CNTs via the formation of continuous and dense CNT stacked structures.However,the viscosities of CNT/polymer blend systems usually increase dramatically with the increase of CNT content,even resulting in serious agglomeration phenomenon due to incomplete wetting.They become not suitable for fiber processing.Direct twisting of CNT aerogels,dry drawing of CNT arrays,and wet spinning of CNT liquid crystals have been demonstrated to be effective strategies for the preparation of high-content or even neat CNT fibers.However,these special techniques rely on high-quality,low-yield,and high-cost CNTs as main fiber-processing materials,such as high length-to-diameter ratio(>10000)CNTs,vertically aligned CNT arrays,and single-walled or few-walled CNTs that can form liquid crystal phases in superacids like chlorosulfonic acid or fuming sulfuric acid by homogeneous dispersing.Multiwalled CNTs(MWCNTs)are easily available in large-scale production at low cost,but they are usually irregular and have low length-to-diameter ratios(<1000),which are too "low-quality" to be processed into high-content CNT fibers by currently reported strategies.Moreover,the phenomena of forming unevenly distributed pores and other defects inevitably occur during the process of assembling CNTs into 1D continuous stacked structures.The densification post-treatment of resultant high-content fibers is usually required for further mechanical performance enhancement.To address the issues and challenges stated above,this study has successfully developed a new and general(dual)polymer assisted acid spinning strategy by taking advantages of the special acid-dissolvable property of aromatic polyamides for continuous production of high-content carbon nanotube fibers based on common MWCNTs with different qualities(such as length-to-diameter ratio and regularity)and strong acids(i.e.,sulfuric acid,95-98%).Thanks to the rational introduction of thermoplastic polymer component,the mechanical performance of obtained highcontent fibers was further enhanced obviously by a unique densification post-treatment of hot pressing with the assistance of elastic surfaces.This special elastic surfaceassisted hot pressing treatment lead to gentle and homogenous densification and effectively alleviate the stress concentration phenomenon caused by pore defects.1.To address the issue of difficulty in continuous fiber formation with low-quality multi-walled carbon nanotubes(MWCNT),we conducted a systematic study of six different spinning solutions.These included untreated MWCNT(U-MWCNT)aciddispersed solution,purified MWCNT(P-MWCNT)acid-dispersed solution,oxidized MWCNT(O-MWCNT)acid-dispersed solution,U-MWCNT acid-dispersed solution assisted by aromatic polyamide(i.e.,poly-p-phenylene terephthalamide),P-MWCNT acid-dispersed solution assisted by aromatic polyamide,and O-MWCNT aciddispersed solution assisted by aromatic polyamide.Our experimental results have demonstrated that neither purified nor oxidized CNT can be continuously spun into fibers.The key to continuous spinning lies in the addition of polymer doping.2.According to the experiment,the mechanical strength of continuous fibers formed using a single polymer-assisted multi-walled carbon nanotube(MWCNT)was limited due to the low length-to-diameter ratio of the MWCNT.This,in turn,made it challenging to collect the fibers continuously.To address the problem of collecting carbon nanotube fibers continuously,we developed a dual polymer-assisted carbon nanotube fiber formation technique.They added varying concentrations(1-8 wt%)of polymers,including polyvinyl alcohol(PVA),to the coagulation solution,which allowed them to create dual-doped MWCNT fibers of aramid and PVA that could be collected continuously.The research concluded that the best force-electrical performance of the dual polymer-doped MWCNT fibers was achieved when the PVA solidification concentration was at 6 wt%.We were able to create fibers with exceptional electrical conductivity and tensile strength at break by using powdered multi-walled carbon nanotubes with an ultra-low length-to-diameter ratio of 100-1000 as the raw material for fiber formation.To broaden our research,we utilized multiwalled carbon nanotubes with different length-to-diameter ratio and from various manufacturers to create the fibers.By using the dual polymer-assisted carbon nanotube fiber formation technique,we successfully produced continuous carbon nanotube fibers,demonstrating the technique’s general applicability through the successful winding and collection of the fibers..3.During the stacking and assembly process of high-content carbon nanotube fibers,structural defects such as unevenly distributed pores are inevitable.To solve this issue,we developed an elastic interface hot pressing treatment that utilizes the thermoplastic repair capabilities of doped polymers.This treatment provides uniform and dense reinforcement,as well as stress relief functions.The morphology,microstructure,and properties of carbon nanotube fibers were systematically analyzed to identify the best conditions for an elastic interface hot-pressing process.The optimal conditions were determined to be 200 MPa and 100 ℃ for 4 hours.Samples produced under these conditions exhibited a significant improvement in mechanical strength and electrical conductivity,with a 2.5-fold increase in mechanical strength and an 8.7-fold increase in electrical conductivity compared to untreated carbon nanotube fibers.The resulting values were 33.4 MPa and 342 S/m,respectively. |
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